The invention pertains to a hydrogen permeation membrane, to a process for the production of hydrogen permeation membranes, and to uses of these membranes.
Metallic membranes which can be used to separate hydrogen selectively from gas mixtures have been known for a long time. To achieve the highest possible hydrogen throughput, thin foils of metals which have high permeability to hydrogen must be used. The metals which can be considered for this purpose are those in which hydrogen has not only high solubility but also a sufficient diffusion capacity. Metals of this type include, for example, palladium and the metals of groups 4b and 5b (titanium, zirconium, hafnium, vanadium, tantalum, and niobium). When they absorb hydrogen, however, these metals become brittle, which makes them unusable for the selective separation of hydrogen.
In contrast, various palladium alloys (such as PdAg, PdCu, PdY) are resistant to hydrogen embrittlement and in some cases have a higher permeability to hydrogen than pure palladium does. For technical applications, palladium alloys, especially PdAg25, are used exclusively for this reason.
In comparison with palladium, however, the metals of groups 4b and 5b have much higher permeation rates, rates which can be one to two orders of magnitude higher. Niobium, for example, has been found to have the highest hydrogen permeability of all (see U.S. Pat. No. 3,350,846 and Journal of Membrane Science, Vol. 85, pp. 29-38, 1993).
In addition to the problem of hydrogen embrittlement, however, the high affinity of these metals for oxygen must also be taken into account, because it leads to the passivation of the surface through the formation of oxide layers. It is therefore also necessary to protect the surface with oxidation-resistant materials such as palladium, which are applied in the form of thin layers ( less than 1 xcexcm) to both sides of the foils. The permeation rate is also significantly affected by the thickness of these protective layers, however, which means that they should be applied as thinly as possible.
Attempts to suppress the hydrogen embrittlement of these metals by various alloying additions have been only partially successful so far. For example, U.S. Pat. No. 4,496,373 describes an alloy with the atomic composition Ti0.5Ta0.3Cu0.1V0 1, in which no hydrogen embrittlement occurs at hydrogen partial pressures of up to 1 bar at T=+300xc2x0 C.
The disadvantage of the hydrogen permeation membranes which are known according to the state of the art and which are suitable for practical use is the fact that it is necessary to process large amounts of noble metals. Because noble metals are expensive, the technical applications must be limited to a handful of specialized areas.
What has been said above leads to the problem of providing novel hydrogen permeation membranes which are able to eliminate the disadvantages described above, at least in part, and of a corresponding process for producing them. In particular, the problem is to provide an affordable noble metal-containing, hydrogen-permeable membrane which undergoes little or no embrittlement on one side and has the same high hydrogen permeation rates as that of the metals groups 4b and 5b on the other side.
This problem is solved according to the invention by means of a hydrogen permeation membrane comprising of a niobium alloy with 5-25 wt. % of at least one element from the group consisting of palladium, ruthenium, rhenium, platinum, gold, and rhodium.
The membrane according to the invention is surprising in that it has very little tendency to become brittle in a hydrogen atmosphere, whereas its hydrogen permeability is greater than that of the corresponding palladium alloys.
It is advantageous, first, for the hydrogen permeation membrane also to contain zirconium and/or hafnium, because these elements considerably reduce the danger of grain coarsening, which promotes the tendency toward embrittlement.
In addition, it has been found advantageous in practice for the membrane to consist of an alloy with 0.5-3.0 wt. % of zirconium and 10-20 wt. % of palladium.
In the process according to the invention for the production of hydrogen permeation membranes, especially for the production of those described above, at least one coating of at least one element from the group consisting of palladium, ruthenium, rhenium, platinum, gold, rhodium, zirconium, and hafnium is applied first to a substrate of niobium or of a niobium alloy with at least one element from the group consisting of palladium, ruthenium, rhenium, platinum, gold, and rhodium to form a composite. Then this composite is subjected to a diffusion heat treatment, so that the individual components diffuse into each other and thus are able to form an appropriate homogeneous alloy.
It has proven advantageous in practice to apply the coating by the PVD process, by chemical deposition, by electrodeposition, or by mechanical plating.
The use of hydrogen permeation membranes according to the invention or the use of hydrogen permeation membranes produced according to the invention as membranes in fuel cells has shown very positive results.
The following examples serve to explain the invention.